新手泛型赋值有关问题！求教

新手泛型赋值问题！求教
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using System.Xml.Serialization;

namespace MangerFT.FS
{
    public partial class WebForm4 : System.Web.UI.Page
    {
        public static string UCFilePath = "D:\\UserConfig.xml";

        protected void Page_Load(object sender, EventArgs e)
        {

        }
        
        [Serializable]
        /// <summary>
        /// 用户信息类
        /// </summary>
        public class Userconfig
        {
            public List<UserDetail> ListUser;
        }

        
        [Serializable]
        public class UserDetail
        {
            private string id;
            /// <summary>
            /// 用户Id
            /// </summary>
            public string Id
            {
                get
                {
                    return id;
                }
                set
                {
                    id = value;
                }
            }

            private string name;
            /// <summary>
            /// 用户名
            /// </summary>
            public string Name
            {
                get

                {
                    return name;
                }
                set
                {
                    name = value;
                }

            }

            private string role;
            /// <summary>
            /// 系统角色
            /// </summary>
            public string Role
            {
                get
                {
                    return role;
                }
                set
                {
                    role = value;
                }
            }

            private bool permission;
            /// <summary>
            /// 扫描枪使用权限
            /// </summary>
            public bool Permission
            {
                get
                {
                    return permission;
                }
                set
                {
                    permission = value;
                }
            }

            private bool unLock;
            /// <summary>
            /// 解锁权限
            /// </summary>
            public bool UnLock
            {
                get
                {
                    return unLock;
                }
                set
                {
                    unLock = value;
                }
            }
        }

        void Serialize()
        {
            Userconfig a = new Userconfig();
            UserDetail b = new UserDetail();

            a.ListUser.Add(b);//这里报错，未将对象引用到实例的对象。。。

            a.ListUser[0].Id = "000000";
            a.ListUser[0].Name = "admin";
            a.ListUser[0].Permission = true;
            a.ListUser[0].Role = "admin";
            a.ListUser[0].UnLock = true;

            a.ListUser.Add(b);

            a.ListUser[1].Id = "000001";
            a.ListUser[1].Name = "admin1";
            a.ListUser[1].Permission = true;
            a.ListUser[1].Role = "admin1";
            a.ListUser[1].UnLock = true;
            
            XmlSerializer xl = new XmlSerializer(typeof(Userconfig));
            xl.Serialize(File.Create(UCFilePath),a);
        }

        protected void Button1_Click(object sender, EventArgs e)
        {

            Serialize();
        }
    }
}

求教
C# 泛型赋值
[解决办法]
 public class Userconfig
        {
            public List<UserDetail> ListUser= new List<UserDetail>();
        }